1. Field of the Invention
The present invention pertains to fluid storage tanks. More particularly, the present invention pertains to storage tanks of the type having a floating roof. Specifically, the present invention pertains to a floating roof storage tank in which the floating roof may be supported at more than one elevation above the bottom of the tank, independently of fluid in the tank.
2. Description of the Prior Art
Many types of fluids are stored in tanks having a bottom, vertical sidewalls and a roof. Many tank roofs are fixed. However, other tanks are provided with floating roofs which cover the stored fluid and ride up and down on the fluid surface therein in response to changes in volume of fluids within the tank. Accordingly, vapor space within the tank is kept at a minimum. This reduces the loss of fluids from evaporation and reduces the hazards associated with vapors, particularly hydrocarbon vapors.
The floating roof of a floating tank is typically constructed of-metal sheets welded together to form a circular deck with a rim around its edge to prevent fluids stored in the tank from escaping from beneath the floating roof. Most, if not all, floating roof tanks are provided with some type of roof support which allows the floating roof to be supported at some elevation above the bottom of the tank when there is little or no fluid therein. There is normally a minimum elevation in which the roof should be supported to prevent it from being positioned below tank inlets or outlets since allowing the roof to do would result in the roof's sinking if additional fluid enters the tank through the inlets or outlets. At other times, it is desirable to support the floating roof at even higher elevations to provide enough room below the floating roof for workers performing, cleaning, painting or other service operations. This allows both personnel and equipment easy access under the roof for performing these operations. In addition, the work may be carried out in a much safer manner.
Typically, storage tanks with conventional floating roofs are constructed with support legs which support the floating roof at a predetermined elevation from the bottom of a tank. Due to the desire to be able to support the roof at some higher position above the tank bottom, floating roof tanks have been constructed in which support sleeves, typically made of pipe, are attached to the roof and extend both downwardly and upwardly therefrom for telescopic engagement with a second smaller diameter pipe. Pins are used to pin the two pipes so that the roof may be supported at different elevations. Of course, this results in several openings through the roof. These openings do not leak liquids due to the support sleeve acting as a well. However, such openings do contribute to vapor loss and subject personnel, repositioning the legs at different elevations, to exposure from vapors and products stored in the tank.
Additional developments have been made in floating roof tanks in which the floating roof may be supported from a fixed roof by support means which are activatable through access openings in the fixed roof. Examples of such may be seen in U.S. Pat. Nos. 3,815,775 and 3,831,800. However, these types of supports require a fixed roof, access therethrough and positioning of personnel on top of the tank for operation thereof. Other developments have been made to support the floating roofs at different elevations by means of rotating the floating roof and alignment of two sets of legs as shown in U.S. Pat. No. 5,305,904. This system requires rotation of the floating roof.